1. Field of the Invention
The present invention relates to an automatic focusing telescope capable of automatic focusing.
2. Description of the Prior Art
Known automatic focusing telescopes include for example one disclosed in U.S. Pat. No. 4,293,187.
In an example of binoculars disclosed in FIG. 1 of this publication, the light ray from an objective lens is divided into a transmitted light ray and a reflected light ray by a semitransparent reflecting mirror composed of a plane-parallel plate so that the transmitted light ray is used as an observation light ray and the reflected light ray is used as a focal point detecting light ray. The transmitted light ray forms an erect image through an erect prism system so that this erect image is expanded and provided for observation by an ocular lens. On the other hand, the reflected light ray from the semitransparent reflecting mirror is directed toward an automatic focusing module so that in accordance with a detection signal of the automatic focusing module, the ocular lens is moved so as to bring the forward focal surface of the ocular lens into coincidence with the focal surface position of the objective lens, thereby effecting the focusing.
Also, in the example shown in FIG. 2 of the above-mentioned U.S. Pat. No. 4,293,187, the light ray from an objective lens is divided into a transmitted light ray and a reflected light ray at a semitransparent reflecting surface provided on an erect prism system so that the branched transmitted light ray is directed to an automatic focusing module.
With the above-mentioned conventional automatic focusing telescope, however, if the plane-parallel plate (the semitransparent reflecting mirror) is arranged midway in an imaging system for the purpose of branching the light ray to the automatic focusing module, the plane-parallel plate is arranged obliquely with respect to the optical axis thus giving rise to a serious problem that a considerable astigmatism is caused to the transmitted light ray thereby making it impossible to observe a sharp image. This problem is particularly manifested in such case where the focal length of the ocular lens is reduced in an attempt to increase the magnification.
On the other hand, where the light ray is divided by the semitransparent reflecting surface formed on the erect prism system without using the plane-parallel plate, the transmitted light ray is naturally refracted greatly. In the case of FIG. 2 in the previously mentioned U.S. Pat. No. 4,293,187, the reflected light ray from the semitransparent reflecting surface is used as a light ray for observation purpose thus causing no problem as astigmatism in the observation system, whereas in the automatic focusing system using the transmitted light ray a considerable astigmatism is caused and the automatic focusing accuracy is deteriorated considerably.